Units question

tokotico · November 8, 2011, 10:55

Hello everybody, I am using pisoFoam to run an external flow over an obstacle, I can not understand the units field in the tutorials where p has dimensions of [0 2 -2 0 0 0 0] instead of what I would expect [1 -1 -2 0 0 0 0].

can anybody point me in the right direction?

Thanks.

Bernhard · November 8, 2011, 11:03

The equations are solved for p/rho, so that rho only appears in the kinematic viscosity. This explains the different units.

tokotico · November 8, 2011, 11:08

Thanks for replying so fast Bernhard,

so the results I get for pressure need to be multiplied by rho to make them Pascals?

¿does it make a difference if I declare units of [1 -1 -2 0 0 0 0] ?

Bernhard · November 8, 2011, 11:10

Yes, then they will be pascals, but be certain from the source code if it is indeed the kind of pressure you think it is.

If you change the units, the dimensions for the different terms are inconsistent. This will throw you a fatal error.

November 8, 2011, 10:55	Units question	#1
tokotico New Member Humberto Join Date: Nov 2011 Posts: 5 Rep Power: 14	Hello everybody, I am using pisoFoam to run an external flow over an obstacle, I can not understand the units field in the tutorials where p has dimensions of [0 2 -2 0 0 0 0] instead of what I would expect [1 -1 -2 0 0 0 0]. can anybody point me in the right direction? Thanks.

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November 8, 2011, 11:03		#2
Bernhard Senior Member Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 21	The equations are solved for p/rho, so that rho only appears in the kinematic viscosity. This explains the different units.

November 8, 2011, 11:08		#3
tokotico New Member Humberto Join Date: Nov 2011 Posts: 5 Rep Power: 14	Thanks for replying so fast Bernhard, so the results I get for pressure need to be multiplied by rho to make them Pascals? ¿does it make a difference if I declare units of [1 -1 -2 0 0 0 0] ?

November 8, 2011, 11:10		#4
Bernhard Senior Member Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 21	Yes, then they will be pascals, but be certain from the source code if it is indeed the kind of pressure you think it is. If you change the units, the dimensions for the different terms are inconsistent. This will throw you a fatal error.